It's not much of a gag when you say "if you happen to use any of these commonplace words I'll modkill you."
Part of the reason captain planet roles have it so easy is because those words get said more often then you might think. >_> There's plenty of figures of speech people use sometimes that include them, and "heart" is really not that weird a word to see in mafia anyway, especially if you're Shadoweh. Now, yes, it's not so often that not being allowed to say them would be much of a limiter, but the issue is more the risk of someone accidentally saying it and then having an incredibly harsh punishment for it. Because if that rule was actually enforced, I think it's pretty likely at some point someone would say one of the words without thinking and get modkilled.
It's more or less a dumb rule because there's absolutely no point in having it (it's not a gag if you're modkilling people for it) and it can distress people and/or really interfere with the game in the event of someone accidentally breaking it.
I mean, I wasn't going to complain (I don't see it as a huge issue, and a weird modkill system on one single game seems fine enough) , but people who don't want to join/are distressed about the rule have very good reason to do so.